This invention relates to internal bone fixation plates.
Clinical experience has established quite clearly that under appropriate conditions, the internal fixation of fractures of long bones of the human body (diaphyseal fractures) with sturdy metal plates is effective in the achievement of fracture healing. However, an undesirable consequence of the rigidity of the internal fixation plates which have been generally used up to the present time is the localized weakening or reduction in strength of the bone which accompanies the protection of the underlying bone from normal stresses after the fracture has been healed. This phenomenon of abnormal rarefaction of the bone is referred to by the medical term "osteoporosis." Following removal of the plate, the weakened bone is vulnerable to refracture and remains so for several weeks or even months, particularly in cases where two plates have been employed. Incidentally, the phrases "plating" and "plated" will frequently be employed in the present specification to refer to bones to which internal fixation plates have been applied.
In the last two decades, the overriding emphasis has been on the development of greater strength of such fixation plates through the provision of heavier plates, and the evolution of concepts such as compression plating. This approach has met with considerable success. However, up to the recent past, this approach has clearly not achieved optimum results, as the long term adverse physiological response of bone to such sturdy devices has been largely ignored.
In recent years, however, considerable interest has been generated in the possible use of less rigid internal fixation plates for fracture management, in view of the fact that the very sturdy compression plates over-protect the underlying healed fracture. In addition to a weakening of the cortex or outer wall the bone, a loss of substance of the periosteum has been noted in tests with dog femurs. Incidentally, the term "periosteum" refers to a specialized connective tissue covering all bones and possessing bone-forming potentialities or capabilities. In addition, incomplete mineralization at the cortex beneath the sturdy metal plate was observed, and bone remodeling and improved strength and increased substance of the bone (including regression of osteopenia, to use medical terminology), started soon after removal of the rigid plate.
Reference is now made to an article entitled, "Potential Application of Graphite and Methyl Methacrylate Resin Composites as Internal Fixation Plates," By S. L-Y. Woo and W. H. Akeson, B. Levenetz, R. D. Coutts, J. V. Matthews, and D. Amiel, Journal of Biomedical Materials Research, Vol. 8, No. 5, September 1974, Pages 321 to 338. In that paper we described a graphite fiber methyl methacrylate composite (GFMM) material which was employed to form softer plates used in the study of fracture healing in dog radii. Traditional sturdy stainless steel plates with tenfold higher axial and flexural stiffness were used as controls. Using combined biomechanical testing and morphometric methods, we noted that at four months postplating, the torsional strength of the whole radius was similar for the composite and the metal plated sides. Morphometric studies using tetracycline labelling techniques showed significantly higher cortical porosity on the metal plated side (14%) as compared to the GFMM plated side (6.3%).
However, the mere selection of softer or less rigid materials to avoid long term adverse effects runs counter to the basic immediate problem of firmly setting the broken bones in their desired relative position. Obviously, if the internal fixation plate is too soft or flexible, a single plate will not hold the bones sufficiently firmly to permit prompt fracture healing.
Accordingly, a principal object of the present invention is to provide an internal bone fixation plate which is not only sufficiently rigid to provide immobilization in the early stages of fracture healing, but is not so rigid as to cause bone deterioration following the initial healing stage.